Process and apparatus for improving clarification of turbid liquids by use of partially thickened sludge



GREEN w. H, 2,429,316- PROCESS AND APPARATUS FOR IMPROVING CLARIFIGATION OF TURBID LIQUIDS BY USE OF PARTIALLY THICKENED SLUDGB Filed Oct 26, 1942 Oct. 21, 1947.

' INVENTOR. 1 WzZZerHGreerz, BY a? e/ I Patented a. 21, 1941 2,429,816 PROCESS AND APPARATUS FOR IMPROVIN CLAKIFICATION OF TURBID LIQUIDS BY USE OF PARTIALLY THICKENED SLUDGE Walter H. Green, Batavia, Ill., Incorporated, Chicago, 111.,

Delaware 'assignor to Infilco a corporation of Application October 2.6, 1942, Serial No. 463,433

12 Claims.

This invention has to do with the'treatment of liquids to clarify them and particularly with the clarification of liquids carrying putrescible matter suchv as sewage.

One of the objects of the invention is to provide an improved apparatus and process for the treatment of liquids'containing suspended solids and colloidal matter, such as sewage and other waste liquors.

Another of the objects of this invention is to provide an improved method and apparatus for clarifying raw sewage, i. e., separating sewage solids from the sewage liquid.

Another object of this invention is to provide a novel process and apparatus for the conditioning of solids, particularly those of a light and fragile,

. type such as sewage solids, for rapid and complete sedimentation, and the subsequent separation of such conditioned solids in the form of a relatively dense sludge, in a unitary apparatus and process.

The process and apparatus will be described in connection with the clarification of sewage for purposes of illustration, but it should not be construed as limited thereto as it will be obvious that the process can be applied to other liquids con-' taining suspended solids with equal facility and benefit.

Raw sewage entering a plant for treatment may 1 pass through a screen to remove coarse particles and rather rapidly through a grit chamber to remove heavier mineral particles that will settle quickly but without permitting the sedimentation of the light and easily putrescible sewage solids. customarily the first step of actual sewage purification will be to pass raw sewagethrough a large sedimentation chamber, or clarifler, for-the deposit of as much of the putrescible solid matter as may be possible. For reasonsthat will be referred to later, such clarifiers are ordinarily made of asize to hold the sewage for a period of about oneand one-half to two-hours based upon the average rate of flow. The sewage flow through .such apparatus is ordinarily continuous although varying in amount at different times, as at different hours of the day. Following such clarifying the sewage is ordinarily passed into a subsequent I purification step for the further removal of organic matter, such step being usually a biological. one.

The size of the clarifler is determined by a compromise between various things. It is, of course, desirable to deposit asmuch as possible of the solids in this clarifier and thereby to relieve the load on the subsequent steps, and this could be accomplished to some extent by making the clarifier larger. If this-is done the cost of the clarifler will be greater, which is undesirable, but more important, the sewage cannot advantageously be retained too long in this clarifying step without putrefaction beginning, with undesirable results in several directions. It is desirable rather to reduce the size of the basin and one of the objects of this invention is directed to this end.

A large part of the solids carried in sewage are light and donot settle readily, so that on the average and with basins designed for one and onehalf to two hour detention of sewage, only 50 or 60 per cent of the-solids will be deposited.

Even the solids that are deposited are mostly quite light, and form a thin sludge. In such apparatus the water content of the sludge is generally up around 97 or 98 per cent, and sludge con t'aining as little as 95 per cent water has perhaps been obtained in a few cases, This sludge must be disposed of some way and the most common way is to subject it to a digestion process in what is known as a digester tank. The size of the digester is determined not so much by the quantity of solids to be digested as by the quantity of the liquid carrying solids that it must receive from the clarifier, and consequently it is desirable to get as thick a sludge as possible from the clarifier. Every advance in this direction permits a correspondingly smaller and less costly digester increase the amountpr proportion of solids reapparatus. If, instead of sending the sludge from a clariiier to the digester for disposal, it is disposed of in some other way, as by being filtered or discharged into a lagoon, it is still desirable to have the water content as small as may be. One of the objects that I have in mind is to increase the density or reduce the water content of the clarifler sludge both by increasing the density of the individual particles forming the sludge and by putting these particles into a form so that they will compact more readily.

'These, and other, objects will be readily apparent from the specification and claims which follow.

Many attempts have heretofore been made to V moved from sewage in the preliminary 'clarifier and some of these have the effect of increasing the density of the sludge, although this may be in part due to the addition of other and heavier solid matter. For example, it is well known that if the sewage is treated with certain chemicals. such as iron salts, or lime, a coagulating efl'ect may be obtained by which particles in the sewage are aggregated and weighted so that advantage '55 can be gained in the way of greater removal,

greater rapidity of settling, and a denser sludge. The disadvantage of such chemical treatment is the cost of the chemicals,- which is considerable if the amount is used that is required to get even approximately the full benefits possible, and this additional cost may exceed the cost of the larger clarifier and digester that might otherwise be required. Chemicals have another disadvantage in that they may have some tendency to stabilize the organic'matter so that it does not so rapidly begin to digest or digestion may proceed moreslowly. I have in mind to use a suitable chemical in my process when it may prove advantageous to do so. However, in most instances the amount of chemical required can be considerably reduced by my process.

v ticles will adhere to each other and thus build up into larger and more settleable particles. Because the flocs so formed are quite fragile, and also to permit the original adherence of the particles, all motion both of the liquid and of the agitating apparatus in the flocculation zone must be very slow as with paddle speeds of more than about one and one-half to two feet per second the result is apt to be destructive rather than beneficial. Because of this slow motion there are not many collisions obtainedin a given space or time, and the flocculation basin has to be rather large for material benefit to be obtained. If the time of retention in the flocculating space is made such as to obtain a reasonably full measure of the benefits that are possible by flocculation, then the flocculation basin tends to become undesirably large. Even though the clarifler may be somewhat reduced in size, the time of retention in the flocculation basin, plus that in the clarifier, is about the same as would be required for a clarifler without thepreliminary flocculation. It has been proposed to combine the flocculation basins with the clarifler itself in various ways, such as using f a part of the space within the clarifler as flocculating space. This has advantages in the way of reducing cost of construction somewhat but doesnot improve the flocculation results nor the timerequired to obtain them, and has the disadvantage that the volume occupied by the flocculator, which ls'desirably very substantial, is subtracted from the volume available for clarification, so that the ,volume of the combined apparatus must be increased in size accordingly, and will tend to beef the same, or greater, dimensions than if the flocculator were omitted.

It is well known in the treatment of liquids to return to liquid entering to be treated a portion of the solids or sludge that have settled from previously treated liquid, such returned solids having a beneficial action in that they tend to weight and enclose newly entering solids and drag the new solids down more rapidly on being allowed to resettle. The solids for this purpose generally have been drawn by a pump, or similar means, from a sludge pocket, into which they were scraped after settling on the floor of a sedimentation basin, or the settling basin might have a sloping floor down which the sludge would flow as it deposits to a pickup point. Such sludge return processes have a beneficial eifect in many cases but in othercases the results have been to so break up the returned sludge that it would not again settle readily and the eflluent liquid from the clarifier has been in some such cases more turbid with the sludge return than without. According to my observation, this result is partly due to the breaking down eii'ect previously spoken of and partly because the sludge brought back is not that which is most desirable for the in-- tended purpose. I have reason to believe that there is always some destructive action in handling and returning sludge in any such way, even though great care be exercised in the handling, and that this effect is cumulative so that while sludge may perhaps be returned a certain number of times without serious damage, yet ultimately the efiect will become decidedly objectionable.

I am proposing to use what might from one aspect be spoken of as a sludge return, but I am proposing to do this in a way that is advantageous and that so far as I am aware has never before been proposed. I propose to use what may well be spoken of as a selective effect as to the sludge or solids that I bring back for reuse, and whatmay be called a cyclic selective circulation of sludge in or through a zone through which entering solids must pass and in which they are gradually changed from a relatively lighter to a "relatively denser condition or sludge. I am aware that selective action in,sludge return has heretofore been proposed but not in the way that I find advantageous and am proposing. Heretofore selective action has been had by allowing the solids to deposit in different zones in some way, whereby there would be obtained a classification effect and the heavier or lighter solids as might be desirable taken from the place of deposit back into a treatment zone. Whatever advantage may be so gained, such methods are open to the objection previously spoken of that in the process of being deposited and picked up some damage results. I propose to use solids that, while they have thickened to some extent,

, are still in a state of suspension or semi-suspension in the liquid and so have not been subjected to any damage that may come from too much thickening or depositing, and which may be brought back for reuse by ordinary means without damage to them. In my process, I use what may well be called "partially thickened sludge in contrast to the return of "thickened sludge" of the prior art. By this means I can bring back the required amount of solids without damage to them, and I secure an additional advantageous eifect. By selecting the sludge or solids in the way I propose I can bring back solids that are of advantageous nature with respect to gathering up the newly entering solids because their surfaces are more closely akin, so that on contact adherence is more likely to occur. Furthermore, I am able to bring back a very large quan-- tity of solids so that a very large amount of surface will be presented thereby and the new solids become, so to speak, absorbed by or incorporated in the volume of older solids as these are distributed through the new liquid. Consequently. it is not necessaryto have the large flocculating chamber that is necessary with the prior art, nor is it necessary to have the agitation that is characteristic of the flocculating process, it being sufflcient to do no more than to provide for the mingling of newly entering liquid containing the new solids and the liquid bringing back the older solids so that contact is obtained. This has the on the sludge so used. It is known that sludge after or upon settling can be caused to thicken by stirring, and I have found that a thickening action upon the sludge can be had, or occurs in carrying out my process, prior to the actual sedimentation of the sludge. Thickening processes maybe the result of either or both of two things. One is the escape of water that is held between particles so that they can come closer-together and the whole bed in this way becomes more compact. Thickening can also be had by means that will cause or aid the escape of liquid from within the boundaries of the particles themselves, which shrink as to size and become more dense. Apparently something of this latter kind occurs upon the continued circulation of what has above been spoken of as partially thickened sludge, but that may be something between a sludge and a true suspension, the material used being of the nature of whatis sometimes spoken of as a semisuspension or a quaking suspension. So far as these particles are of the nature ofgels they are subject to synerisis and this tendency to shrink is apparently increased when they are disturbed as they are in being carried around in my process. Whatever may be true in such respects, by the circulation I am about to describe, more rapid clarification and a denser sludge may be had.

I have in mind also a further object and result in that in the circulation I propose part of the liquid in the apparatus and part in the sludge is kept in an aerobic condition which tends to prevent the development of a putrescent condition and which may have some advantageous effect in promoting the clarification and thickening process.

tom, the lower edge being a-suitable distance below the water level inthe tank so that the liquid escaping thereunder will become clarified in passing to the launder 1., In the'center of this cylinder II is a tube or inner cylinder I2 adJust-' ably supported from the beams III as by the rods I3 having at the upper end a thread and nut, generally indicated by I4. To the bottom of the inner cylinder I2 is attached a plate, or baflle I5 of diameter'somewhat greater than that of the inner cylinder I2,- plate' I5, being spaced away from the bottom of the cylinder a distance to provide for the inflow of sludge into the cylinder. Preferably, the baifle I5 will be adjustably mounted, as by rods I8, also provided with a thread and nut II at the upper end thereof, so that the spacing between the plate or baflie I5 and the inner cylinder can be adjusted. The bottom I8 of the inner cylinder I2 is open and is placed at an intermediate level of the basin I. The upp r end I9 of the inner cylinder is also open and isplaced below the liquid level in the tank, or basin, so that anupward flow of liquid through the inner cylinder can be'readily maintained. The outer cylinder I Iis also open at both ends, theupper end 2 bein above the liquid level of the basin so as to provide a wall which, at the surface of the liquid separates the flow coming from the inner cylinder I2 from the quiescent clarifying zone lnthe upper portion of the basin I; and the lower end 2| being at a level substantially below the liquid surface, but above the level 'of the lower end I8 of the inner cylinder I2. This construction provides a mixing chamber with an inlet I8 at a lower intermediate level and anoutlet 2I at a higher'intermediate level. It is necessary that the outlet H .be substantially below the liquid surface so that the upper portion of the liquid in the basin will be maintained in a quiescent condition in order that liquid rising therein may become clarified.

It is also preferable that inlet I8 and outlet 2I be at diflerentlevels, with the outlet placed above the inlet, for reasons which will become apparent later.

With such and other objects in mind, my inbe of any suitable size and construction, and

similar in construction to the clarifiers, or sedimentation basins of the sewage treating art.

Such clarifiers may be of any suitable shape, and generally include vertical side walls 2, and a floor 3, which preferably is inclined somewhat toward the center, as shown. Adjacent the center of the bottom is shown a sump 4 from which leads a discharge pipe 5 provided with a valve 6 for the removal of sludge. Adjacent the top of the basin is launder I from which leads the eflluent pipe 8 for the discharge of clarified water.

The tank is spanned by beams Ill from which depends an outer cylinder I I open at topand bot- Adjacent the bottom 3 of the basin I is shown a scraping mechanism 25 of common form, this being carried on a'shaft 26 and driven by a motor 21 equipped with a speed reducer 28, the scraping mechanism and drive being carried by the beams III in any suitable manner. Also driven by the motor 21 and reducer -28 is a quill'shaft 29 which extends into the inner cylinder I2 andhas a pump, or propeller 30, rigidly secured thereto. The two shafts 26 and 29 are necessary as it is well known that the scraping mechanism 25 must be driven very slowly, while it is desirable that a substantial flow of liquid be pumped through the inner cylinder. I2 so that it is desirable that the pump rotate quite rapidly. Sewage to be treated enters through the inlet pipe 35, discharging as at 36 into the inner cylinder I2, pref v erabiy. at a point adlacent'propeller 30..

Operation of the apparatus and practice of the process is as follows; Assuming the basin Ito have been filled with sewage, the quill shaft 28 and propeller 30 are rotated to draw liquid in over I plate I6 into inner cylinder l2, from. which it will pass into the outer space enclosed by cylinder II and then downwardly. Raw sewage en-.

tering through the inlet pipe 35 will be intro-- duced into the. rising column of circulating liq- This liquid will pass outwardly from the lower end 2| of the outer cylinder ll, and being of a across the basin. which result because the discharge at the lower end II of the outer cylinder ii is heavier than the more clarified liquid above, is advantageous in several respects, among them that it reduces or eliminates the tendency for short circuiting from the outlet II to the launder 1 that occurs to a greater or lesser extent in most cases. Part of the liquid passed down through the outer cylinder ii will perhaps be drawn down directly to the bottom end of inner cylinder i2 and reenter, but since the liquid reaching the bottom of outer cylinder ii and discharging out into the basin-has been diluted with the raw sewage it will tend to be lighter than the suspension or thickening sludge below that point, andv the tendency will be for a large portion, and perhaps most of the liquid emerging from the outlet 2i to spread across 'the tank to the level as referred to above. Due to the same cause, the liquid, or partially thickened sludge, from a lower level will be drawn in across plate it into the lower end i8 of cylinder i2. Thus, in general, the circulation due to the action of the propeller 30 is not confined to the space enclosed by the cylinders and the baflle plate i2, but, because of gravity efiects, includes a depth or layer over the whole area of the tank betweenthe level of the lower edge 2] of the outer cylinder ii and. the baffle plate i5, and in this whole volume the actions that result in clarification and thickening can, and do, take place.

It'ls to be noted also that since this whole volume of liquid within the zone of circulation, and it is a substantial portion of the contents of the clarifier, is repeatedly brought back up to the surface and exposed to atmosphere within the outer cylinder ii, a degree oi aeration .ismaintained that is desirable for various reasons including that of tending to prevent or lessen septic conditions. It is partly because of the use oi the space within the basin in the zone between the levels of the lower end ii of the outer cylinder II and plate ii in the way'described that the. volume within the space indicated by the outer cylinder Ii and the baille plate l may be made quite small, and thus the effective volume of the tank I is not materially reduced. Actually, due to the increased rapidity of clarification and the aid to thickening resultingfrom the described process, actual size of the basin I may be reduced.

As previously referred to, the inner cylinder l 2 is adjustably supported on the beam i0, so that it can be raised and lowered and with it, or independently, the plate i5 moved up and down. The purpose of this adjustment i to provide some selection as to the .Jevel at which solids or sludge is drawn into the mixing zone from the space'within the basin i. It is not essential, although it may be desirable, that the cylinder l2 itself be adjustable, but the baille plate I 5'should be supported in some way so that it will be adjustable.

Solids collecting on the floor 3 of the tank i are scraped into the sump 4 for discharge by retation of the scraper 25 by the motor 21 and this scraping and discharge canbe continuous or pethe particular case.

may be indicated in It will be observed tho-t the process here de- 8 riodic as is desired and as scribed is something essentially diilerent iron the coagulation or flocculation of sewage or other turbid liquid as heretofore proposed. There is no reliance on the agitation that is characteristic of and essential to the flocculation processinstead reliance is placed upon mixing oi the two flowing streams, to secure entrapment of the new sewage solids by the partially thickened sludge. The apparatus lacks the movable paddles that are also characteristic of and essential to the flocculation process, there being required only the propeller or other suitable pumping means for causing the circulation from the basin into and through the mixing space. above, I rely on sedimentation and my process can be emciently practiced in an ordinary clarification basin. While I rely on the particular character oi the partially thickened sludge, and mix it with raw sewage, the resulting mixture is subjected to quiescent sedimentation. The differences in specific gravity provide the selective return which my process requires. 0

Since the liquid levels within the cylinders are preferably the same as in the tank i little power is required for this purpose. It should be remembered, however, that while the agitation of the flocculation process is not required here, yet the particles or solids resulting from this method of treatment have, in addition to other desirable characteristics referred to, the additional characteristic of tending to be tough so that the velocity of flow used within the mixing space and the speed of the propeller 30 may be higher than the speeds used in'fiocculation processes. Thus, with my process the speed at which the impeller 30 is rotated is relatively unimportant. It should be at such a velocity as will provide the required flow of partially thickened sludge, but can be fast or slow as desired.

It will be apparent that the mixture oi raw sewage and partially thickened sludge passed through the inner cylinder l2 and the outer cylinder i i will begin to deposit as sludge immediately upon being removed from the positive circulation within the two cylinders. The depositing sludge will gradually become thicker until at the floor 3 of the tank, it will become quite thick for a sewage sludge. It will be obvious that the material brought into the inner cylinder i2, which will be above the level of the baiile plate i5, will contain solids which are in the process of depositing, but have not yet come to a state of rest. It is these settling solids that I refer to in the term ing upon conditions of operation, that is, upon the quantity of circulation in comparison to the throughput of sewage, the material returned to the inner cylinder i2, for mixing with raw sewage may be either a true suspension or a deposited sludge, but-it is preferred to use the partially thickened sludge as above described.

The volume of recirculating partially thickened As indicated sludge should be at least equal to the volume of entering raw sewage, in order to provide a substantial quantity of recently aggregated solids for contact with the solids contained in the raw,

of said sewage outlet and the lower end of which is spaced above the bottomof said tank, an outer upper end of which isabove the'ievel of saidturbulent pumping as it is found that such can reach a point at which it becomes destructive to the partially thickened sludge. It will be found that when the throughputof sewage is properly balanced with circulation through thecylinders comprising the mixing zone, that the sludge in the outer portion of the tank tends to stratify into varying degrees of thickening sludge. In the extreme upper part of the tank is clarified sewage; below that. adjacent the outlet 2|, from the mixing zone, the emerging liquid will be a true suspension; below that, but still in the intermediate zone, we find the partially thickened sludge, while at the bottom will be found the fully thickened sludge which has come to rest upon the floor of the basin. I have found that, as indicated above, the return of the fully sedimented sludge is undesirable in several respects. However, the use of the partially thickened sludge avoids defects of the old sludge return process and aids materially in the processing of sewage solids for sedimentation. The baille plate I5 is used to secure the return of the partially thickened sludge, but to prevent the return oi. the thickened or fully settled sludge. This baiile therefore, has

two functions, (1) of fixing the lower level'of withdrawal and (2) to prevent the disturbance chamber surrounding said inner chamber, the

sewage outlet and the lower end of which is below the level ofsaid sewage outlet and above the level of the lower end of said inner chamber, a pumping device in said inner chamber operable to cause flow therethrough and through said outer chamber, a horizontally extending baiiie below and adjacent the lower end of said inner chamber, and afisewage inlet opening into the inner chamber.

5. Ina liquid treating apparatus comprising a tank, an inlet into said tank, a treated liquid outlet from the upper portion of said tank, a solids outlet from the floor of said tank, and means for moving solids deposited on the floor of said tank toward said solids outlet, the combination which comprises a vertically extending partition means in said tank forming therein a mixing chamber,

said mixing chamber being so located that the inlet for sewage to be clarified discharges thereinto,.an outlet from said mixing chamber opening into said tank at a level below the level of said treated liquid outlet, an inlet into said mixing chamber from the interior of said tank located at a level below that of said mixing chamber outlet and a substantial distance; above the solids outlet, a horizontally extending baffle intermediate and cons quent resuspension of the fully thickened sludge.

I claim:

1. In a liquid treating apparatus comprising a tank, means for delivering liquid to be treated into said tank, an outlet for treated liquid from the upper portion of said tank, a solids outlet from the floor of said tank, and means for moving solids deposited on the fioor of said tank toward said solids outlet, the combination which comprises a vertically extending wall structure in said tank forming therein a mixing chamber, said chamber having an inlet opening from and an outlet opening to the interior of said tank at intermediate levels therein,.said outlet opening being at a level below the said treated liquid outlet and the inlet opening being at a level below that of said outlet opening, a horizontally extending baffle below the inlet opening to said mixing chamber and of greater diameter than said inlet opening, and a pumping device operable to cause fiow through said mixing chamberfrom said inlet opening to said outlet opening, .said means for delivering liquid to be treated into said tank being positioned to deliver liquid to said mixing chamber.

2. The apparatus of claim 1 wherein the elevation of the inlet opening to the mixing chamber is vertically adjustable.

3. The apparatus of, claim 1 wherein the horizontally extending bailie is vertically adjustable.

4. A sewage clarifying apparatus comprising a tank, a clarified sewage outlet from the upper portion of said tank, a sludge outlet from the lower-portion of said tank, means for moving sludge settling on the floor of the tank toward said sludge outlet, 2. vertical inner chamber in said tank, the upper end of which is below the level said mixing chamber inlet and the solids outlet, and a pumping device operable to cause flow through said mixing chamber from said mixing chamber inlet to said mixing chamber. outlet.

6. In the process of clarifying sewage which includes maintaining a large body of sewage undergoing clarification, said body of sewage having- I a zone of clarified liquid in its upper portion, a

zone of thickened sludge in its lower portion and a zone of partially thickened sludge intermediate said first mentioned zones, withdrawing clarified sewage from the upper portion of the clarified liquid zone and removing deposited solids to waste from the lower portion of said zone of thickened sludge, the improvement whichcomprises mixing partially thickened sludge from said intermediate zone and sewage entering to be clarified in a partially confined mixing zone, and discharging the mixture from the mixing zone into an upper level of said zone of partially thickened sludge.

'7. The process of clarifying sewage which comprises retaining sewage while undergoing treatcentral elevation and substantially below the surface of the sewage in said clarification zone, withdrawing clarified sewage from the upper portion of the clarification zone and removing deposited solids to waste from the lower portion of the clarification zone.

8. In. .the process of clarifying sewage which includes maintaining a large body of. sewage undergoing clarification, said body of sewage being comparatively clear at the top and of increasing solids density downwardly to the form of a thickened sludge at the bottom, withdrawing clarified sewage from the upper portion of the body of sewage and removing deposited solids to waste ,from' the lower portion of the body of livering sewage entering-to be clarified into a relatively small and partially confined mixing zone within said body of sewage, withdrawing partially thickened sludge from an intermediate Lamas sewage, the improvement that comprises do- I portion of said body of sewage spaced above the stratum of sludge of increasing solids density downwardly including respectively light sludge, partially thickened sludge and finally thickened sludge at the bottom of said zone, passing sewage to be clarified into said quiescent zone, withdrawing clarified sewage liquid from the stratum oi clarified liquid and removing thickened sludge.

from the bottom of said zone, characterized by the steps of mixing entering sewage to tie clarifled with partially thickened sludge taken from an intermediate elevation of said stratum or sludge spaced above the bottom thereof a distance greater than the depth of thickened sludge and discharging said mixture into said stratum of sludge at a higher elevation, whereby the settleability of solids in the sewage is improved.

10. In combination with the clarification oi sewage wherein the sewage is passed through a sedimentation zone, clarified sewage is withdrawn from the top of saidzone, and thickened sludge from the lower portion of said zone, the improvement which comprises continuously withdrawing predetermined amount of partially thickened sludge from a selected intermediate level within said sedimentation zone spaced above the floor of said sedimentation zone a distance greater than the thickness of thickened sludge in the bottom of said sedimentation zone, mixing said withdrawn partially thickened sludge with the sewage entering to be clarified, and discharg-' '12. In the process of clarifying sewage wherein sewage to be clarified is passed into and through a sedimentation none in which solids deposit to form a sludge oi downwardly increasing density, thickened sludge is collected in the lower portion of said zone and removed to waste therefrom, and clarified sewage is withdrawn from the upper portion of said zone, the improved method 01' conditioning sewage solids for sedimentation which comprises mixing the incoming sewage to be clarified with partially thickened sludge from an intermediatelevel of said zone, agitating the mixture of incoming sewage and partially thickened sludge in a confined mixing zone, and discharging the agitated mixture into another intermediate section of the sedimentation zone.

WALTER. H. GREEN. Y

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